The beam-formers are used in the multiple-beam antennas to generate output beams from input radiofrequency signals. A conventional beam-former comprises N inputs In1 to InN, P outputs Out1 to OutP, and a plurality of radiofrequency circuits 11, 12, 13 suitable for dividing and recombining the input radiofrequency signals according to a phase and amplitude law chosen to form output beams. There are various beam-former technologies. In FIG. 1, the radiofrequency circuits comprise a large number of individual waveguides 10 which cross over one another so as to allow combinations necessary for the formation of the various output beams by the radiofrequency signal combiners 12. These beam-formers are suitable for a limited number of radiating elements and for forming a limited number of beams because they become very complex when the number of beams increases because of the necessary crossovers between the waveguides.
It is also known practice to form beams by using a Butler matrix consisting of a symmetrical passive circuit with N input ports and N output ports, which drives the radiating elements producing N different beams of equal amplitudes. The circuit is made up of junctions which connect the input ports to the output ports by N different and mutually parallel transmission lines 18. There are a number of possible Butler matrix configurations. In the diagram of FIG. 2, the Butler matrix comprises couplers 15, of 3 dB, 90° hybrid coupler type, making it possible to combine or divide the power of the input radiofrequency waves, phase-shifters 16 suitable for applying a phase delay of 45°, and crossover devices 17 making it possible to cross over two different transmission lines. As is known, each crossover device 17 can consist of two 3 dB, 90° couplers connected in series. An example of Butler matrix architecture with four input ports A, B, C, D and four output ports A′, B′, C′, D′ is represented in FIG. 2. In this example, the Butler matrix comprises four 3 dB, 90° couplers, two 45° phase-shifters and a crossover device. This type of beam-former is well suited to the formation of a small number of beams but becomes too complex when the number of beams increases. Furthermore, it allows for the formation of the beams only in a single direction of space at right angles to the transmission lines 18.
According to another technology, there are planar quasi-optical beam-formers that use an electromagnetic propagation of the radiofrequency waves originating from a number of feeders placed at the input, for example feeder horns, according to a generally TEM mode of propagation between two parallel metal plates. The focusing and the collimation of the beams can be performed by an optical lens as described, for example, in the documents U.S. Pat. No. 3,170,158 and U.S. Pat. No. 5,936,588 which illustrate the case of a Rotman lens, or alternatively via a reflector as described for example in the documents FR 2944153 and FR 2 986377, the optical lens or, respectively, the reflector being inserted on the propagation path of the radiofrequency waves, between the two parallel metal plates. Different types of optical lenses can be used, these optical lenses serving essentially as phase correctors and making it possible, in most cases, to convert one or more cylindrical waves emitted by the feeds into one or more planar wave propagating in the parallel metal plate waveguide. The optical lens can comprise two opposing edges with parabolic profiles, respectively input and output. Alternatively, the optical lens can be a dielectric lens, a graded index lens with straight edges, or any other type of optical lens. In the case of a quasi-optical beam-former with optical lens, to obtain a planar antenna, it is sufficient to place input radiating elements around the input edge of the optical lens and to fix radiofrequency probes on the output edge of the optical lens, then to link each radiofrequency probe to an output radiating element via a transmission line, for example a coaxial cable. In the case of a pillbox beam-former, to obtain a planar antenna, input radiating elements are placed in front of the incorporated parabolic reflector, and output radiating elements are placed on the path of the radiofrequency waves reflected by the parabolic reflector. There are various pillbox beam-former solutions, using one or more reflectors.
Since this technology uses parallel plate waveguides, as an alternative to the use of a number of discrete radiating elements aligned side-by-side, it is possible to use a continuous linear aperture at the output of each parallel plate waveguide. These linear apertures, which are not spatially quantified, have performance levels very much superior to the linear networks of a number of radiating elements, for the beams that are misaligned, because of the absence of quantization, and in terms of bandwidth because of the absence of resonant propagation modes.
A quasi-optical beam-former is much simpler to produce than the traditional beam-formers with individual waveguides because it comprises neither coupler, nor crossover device. However, all the known planar beam-formers are capable of forming beams only in a single dimension of space, in a direction parallel to the plane of the metal plates. To form beams in two dimensions of space, in two directions, respectively parallel and orthogonal to the plane of the metal plates, it is necessary to orthogonally combine together two beam-forming assemblies, each beam-forming assembly consisting of a stacking of a number of unidirectional beam-forming layers. To orthogonally combine two beam-forming assemblies, it is further necessary to form connection interfaces, in particular input/output connectors, on each beam-forming assembly, then to link two-by-two, the various corresponding inputs and outputs of the two beam-forming assemblies by dedicated interconnecting cables as represented for example in the document U.S. Pat. No. 5,936,588 for lens-based beam-formers. This architecture is satisfactory for the formation of a small number of beams, but becomes very complex and excessively bulky when the number of beams increases.
To our knowledge, to date, there is no planar beam-forming device that makes it possible to form beams in two dimensions of space. Nor, moreover, are there any simple solutions for interconnecting two unidirectional beam-formers making it possible to dispense with the connection interfaces and interconnecting cables.